The present invention relates to an improvement of a ceramic heater and particularly it is applied to a heater for the regeneration of an apparatus for collecting and purifying fine particles contained in the exhaust gas of a diesel engine or the like.
For collecting and purifying fine particles contained in the exhaust gas, it has been proposed to use a heat-resistant honeycomb- or foam-like filter made of a ceramic material and fire and burn the collected fine particles by means of a heat-resistant ceramic heater disposed on an upper surface of the said filter to regenerate the latter. In this case, from the standpoint of easiness of handling of electrodes, prevention of heater breakage, protection of electrode portions and unification of the heater shape, it has been considered desirable to dispose a plurality of plate-like heaters formed in the shape of U or V.
Usually, a U- or V-shaped heater is connected at its both end electrode portions 35 to metallic lead wires for example as shown in FIG. 7 and is thereby charged electrically and heated, so with a view to protecting the electrode portions the sectional area of the electrode portions is made fairly larger than that of a heat generating portion to suppress the heat generation at the electrode portions.
However, in a conventional ceramic heater there arises an opening/closing motion of the leg portions in the course of heat generation and cooling due to thermal expansion, and once the heater is fixed to a holding portion, the above motion is impeded by friction, for example, and stress is concentrated on a bent portion 25a. This stress concentration of the bent portion 25a is proportional to a moment of an external force exerted on the leg portion and a length (Lo) from the leg portion to the bent portion 25a. Consequently, this shape causes the problem that stress is concentrated on the bent portion 25a and a repeated supply of electricity results in crack of the bent portion.
The above conventional ceramic heater also involves the problem that when it is disposed on the filter surface, it is impossible to heat the filter surface widely because the area of heat generation is small in comparison with the large space of the electrode portions.